Coal-fired power plants, or incineration plants generate high-temperature gas containing a large amount of fly ashes and dusts, sulfides, nitrides or other contaminants, and if the exhaust flue gas or raw syn-gas is emitted without processing, the environment of mankind will be affected seriously. In order to solve the environmental pollution problems due to hazardous gases, industrial countries have to constitute strict emission standards, and also input considerable research and development (R&D) resources to investigate how to remove the harmful pollutants in the gas streams effectively so as to conform to the emission standards.
In prior art, many methods of gas cleanup exist, among which some methods are common, namely mechanical separation of dust in cyclones, electrostatic dust collecting, baghouse filters, ceramic candle filters, granular moving-bed filters, etc. Cyclones are considered to be powerful and cheap pre-separators for gas cleanup purposes. Their removal efficiency is, however, limited to about 90% and rapidly deteriorates for particles smaller than 10 microns.
In the most widely used electrostatic dust precipitators, corona is utilized to ionize the exhaust gas so as to make the harmful materials carrying negative charges, and these materials are then caught on earthed collector plates to achieve the objective of gas cleanup. Electrostatic precipitators can be operated economically in flue gases of large volumes, but their efficiency is influenced by effects of chemical composition of particles, particle electric resistivity, moisture content, and temperature of gas.
Furthermore, another baghouse filter dust collecting technique also achieves the effect of gas cleanup through retaining the harmful materials in the exhaust gas, passing through the baghouse filter fabric. The baghouse filters offer very high dust collection efficiency and, operating in low temperature, they have the advantage over electrostatic dust precipitators, that the electric resistivity of dust particles does not play any role, making them competitive for high-resistivity ashes. Particles of different size are removed by different physical mechanisms in baghouse filters. The highest removal efficiencies are obtained for the large particles at high gas velocities and for the finest particles at low velocities. Baghouse filters can work at higher temperatures, depending on fabric materials. Ceramic materials, usually based on alumina, quartz or aluminum silicates are the best choice. A disadvantage when compared with electrostatic dust precipitators is the larger pressure drop and relatively low gas face velocity. Low gas face velocity gives rise to large filtration surface and inherently high costs.
Ceramic candle filters have been and are still being tested at full scale operation at several IGCC demonstration projects. Filter cleaning is done by backpulsing usually with nitrogen. Typical problems encountered during the testing are the breakage and strength degradation of ceramic candles owing to the overheating, thermal shocks and excessive pressure drop. In addition, the plenum vibration and back-pulse cleaning expose the candle filters to thermal and mechanical fatigue stresses that may ultimately lead to fracture of the filter elements.
Granular moving-bed filters are very well suitable for high temperature gas filtration. They are developed as key subsystems of current integrated gasification combined cycle (IGCC) and advanced pressurized fluidized bed combustion (PFBC) power generation systems. Their potential economic advantage is based on the fact that the granular moving-bed filters may be more compact than either electrostatic precipitators or baghouse filters. The principal disadvantage of these filters is that either a very thick bed or very fine granular material (or both) are required to give high removal efficiency of particulates in the 0.5 to 10 microns size range. The requirement for thick beds results in large expensive equipment, while the use of very fine granular material causes high pressure drop, poor bed flow characteristics, accompanied by stagnant zones and hot spots in granular moving bed, causing corrosion and plaques on louver wall. Flow patterns in moving bed are influenced by friction of filter media on louver walls. It results in uprise of stagnant zones alongside louver walls. Besides particulate removal, granular moving beds may be able to capture other contaminants through the use of other materials (such as sorbent, activated carbon, etc.), in two-stage process, but in one apparatus.
In order to improve the removal efficiency of granular moving-bed filters, and extend the options in which the granular filter media can be applied, the technologically developed countries are devoted to research and development of environmental-friendly gas cleanup technologies, and so far, the granular moving-bed filter is a highly regarded gas cleanup apparatus, and can be considered as the exhaust gas cleanup apparatus widely used in the future. FIG. 1 is a schematic view of a common granular moving-bed filter. Referring to FIG. 1, the granular moving-bed filter is constituted by a plurality of hopper-shaped structures 16, and each hopper-shaped structure 16 has louver walls 10 and 11 at two sides thereof. The granular material, serving as the filter medium, enters from the top of the granular moving bed, so that the granular material 15 goes down along the hopper-shaped structure 16 to clean up the harmful substances in the exhaust gas stream passing through the granular material, and the granular material which has adsorbed impurity or lost filtering effect exits from the bottom of the granular moving bed. In FIG. 1, the raw syn-gas or exhaust gas flow 90 enters the granular moving bed from the louver wall 10 at inlet side of the granular moving bed, and passes through the filter medium 15. The harmful or polluting material contained in the exhaust gas flow 90 is filtered out by the filter medium 15, and the clean gas flow 91 exits from the louver wall 11 at the outlet side of the granular moving bed.
In order to enhance the efficiency of process in conventional single-stage granular moving-bed apparatuses as the one shown in FIG. 1, a two-stage gas-solid moving-bed concept was evolved, wherein the depth of moving bed is divided between two stages, accommodated in two separate vertically mounted units (e.g. an absorption device shown in U.S. Pat. No. 4,349,362) or where single moving-bed filters (units) arranged horizontally act in series, such as a filter shown in FIG. 2 which is a device for the separation of dust from hot gases disclosed in EP 0321914. In the filter of FIG. 2, dust-laden hot gases are directed perpendicularly through vertically arranged bulk layers A, B, C in sequence so as to be purified by the filter materials contained in the three bulk layers A, B, C in a multiple-stage process.